This invention relates to a developer roller, and a method of making developer rollers used in image forming devices such as laser printers, photocopiers, or facsimile machines. Many of these image-forming devices make use of the same basic technology. These devices make use of a substance known as toner to print images on paper or other media. Toner is a fine dry powdery material. Toner is electrically chargeable, and may be magnetically attractable. The toner is typically contained in a section of a toner cartridge called the toner hopper and is transferred from the toner hopper toward a photoreactive drum or belt that contains a latent image. The latent image is an area of the drum that has a changed electrical charge in the shape of the image to be formed. Toner is electrically attracted to the latent image. The toner in the form of the latent image is then transferred to a sheet of paper or media and then fused on to the paper or to the media.
The toner is transferred from the toner hopper to an area adjacent to the photoreactive drum or belt by a developer roller. Developer rollers maybe used with both magnetic and non-magnetic toners. The most common type of the developer roller is the magnetic development roller and is used in the popular Hewlett Packard Laser Printer series.
A magnetic development roller is used with toner which is both magnetically and electrically attractable. An early embodiment was described in U.S. Pat. No. 3,219,014 issued to Mott et al. These developer rollers are generally rotatable hollow cylinders surrounding a fixed permanent magnet. These rollers are made of a variety of materials but all development rollers have needed to exhibit certain characteristics no matter what their make-up. First, the surface of the development roller must be electrically conductive. Second, the roller must have a texture so as to tribo-charge and carry the toner. The friction of the moving developer roller against a doctor bar or similar device imparts an electrical charge to the toner through tribo-charging. Third, the developer roller must be durable enough to maintain good performance throughout the useful life of the toner cartridge. Fourth, the developer roller must have good release characteristics so that the toner on the developer roller will leave the developer roller and go toward the latent image on the photoconductive drum. Fifth, in response to the electrical attraction, if the developer roller is a magnetic development roller, the developer roller should be magnetically transparent so as not to interfere with the magnetic field generated internally to the developer roller. The key problem in making a developer roller is balancing these qualities of durability and performance. Solutions for these characteristics include using a metallic developer roller such as that described in U.S. Pat. No. 5,749,033 to Swartz et al. Such a developer roller is electrically conductive, durable, has release characteristics, and may be textured by blasting the substrate. Others have used electrically conductive plastic materials as the surface of developer rollers. One such roller is described in U.S. Pat. No. 5,659,862 issued to Mahmud and another in U.S. Pat. No. 4,989,044 issued in Nishimura.
These prior art developer rollers have not fully achieved the release characteristics of the toner allowing the toner to move readily from the developer roller to the photoconductive drum or belt. By improving their release characteristics, toner transfer efficiency can be improved resulting in a better print with a lesser consumption of toner. With respect to the plastic coated developer rollers, great care must be taken to ensure uniform electrical conductivity at the surface of the developer roller together with uniform volumetric resistance below the surface. In addition, the prior art plastic coated rollers have used Phenolic resin as the binder material. Phenolic resin is friable and wears readily. The improved method producing the developer roller and the developer roller produced thereby described herein meets and solves these problems and is extremely efficient at transferring toner while maintaining a high degree of durability.
The developer roller of this invention has an electrically conductive substrate surrounded by multiple layers of a polymer containing finely ground graphite and wetting agent. These layers are spray applied in multiple passes. In a preferred embodiment the polymer graphite wetting agent mixture is diluted in a solvent until the solids are approximately 5% by weight of the spray. The wetting agent encourages even dispersion of the polymer graphite wetting agent mixture. By spraying in multiple fine layers good adhesion between layers is promoted. The use of a wetting agent encourages dispersion of the graphite uniformly throughout the material providing for better electrical characteristics, and the use of the wetting agent also promotes adherence between each sprayed layer and between the polymer-graphite-wetting agent layer and the substrate. Use of the wetting agent, therefore, provides for a more durable developer roller. A final top coat of essentially all graphite may be added to the polymer graphite wetting agent mixture. This final coating provides an extremely conductive, extremely lubricous coating and provides the qualities of toner release that are desired. This final coating is dispersed in a solvent, preferably isopropyl alcohol and spray applied. Again, the use of the wetting agent ensures good adherence between the graphite on the surface and the polymer-graphite-wetting agent layer below. The graphite wetting agent layer can then adhere to the graphite in the polymer and to itself creating an essentially all graphite surface that is maintained during the useful life of the developer roller. Multiple layers of the graphite wetting agent layer may be applied. After the essentially graphite and wetting agent layer is spray applied in multiple coats it is burnished with a goat hair brush or similar material to ensure that the graphite and wetting agent are pressed into the pores of the polymer material further promoting adherence. The burnishing may be done once, or multiple times as layers of graphite wetting agent are applied. The method making the developer roller described in this invention is easily automatable, uses readily available materials, and produces a durable developer roller with superior print characteristics.